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assumption about a device's maxphys

hi thor,

your tls-maxphys branch appears to assume that a given device's maxphys is 
but consider the case of a software "device" like a logical volume from LVM.
one very nice feature of a volume manager that I used to work with (veritas 
is that it can migrate a volume to new storage on the fly, while the data in it
is being read and written.  with such a virtual device, I assume that intention
would be that the maxphys of the volume would be the same as the maxphys of the
underlying physical device, but the maxphys of the underlying physical device
could be different before and after a volume migration, so that doesn't fit
very well with the assumption that a device's maxphys doesn't change.

caching "the" device maxphys in a file system's "struct mount" is also 
since a file system may directly access multiple underlying devices, as ZFS 
and those devices may each have a different maxphys.

could you think about how to enhance the new maxphys design to accomodate these 


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